Method and apparatus for removing prosthetic cement

ABSTRACT

A device, and method of producing and using same, for use in removing the prosthetic cement from a bone in which an artificial joint (such as hip joint) is to be replaced. The device cuts the prosthetic cement in a longitudinal plane of the prosthetic cavity to the complete depth or the cement but without substantially cutting into the bone. The device includes a reciprocating saw head (8) which is advanced longitudinally of the cavity and maintained within the preselected longitudinal plane of the cavity by a guide (1) while it cuts through the cement (10). The saw head (8) is transversly biased towards the cement so that it cuts only to a depth defined by the profile of a template (6). The profile of the template (6) is obtained from X-ray information obtained before commencement of the operation.

BACKGROUND ART

The present invention relates to a method and apparatus for the removalof prosthetic cement from the bone of a patient undergoing total jointreplacement, particularly total hip joint replacement.

Some prostheses, used in total hip replacements especially those used inearly procedures, included long stems which were embedded in prostheticcement within a suitable cavity within the bone. Loosening of suchprostheses requires that they be removed and replaced. Many suchprostheses last approximately five to ten years before loosening. Themajor problem in the removal and replacement of the original prosthesisis found in the removal of the prosthetic cement which is normallypolymethylmethacrylate.

The removal of the cement is usually effected with hammer and chisel.The surgeon is working within a live bone, down a narrow curvedcylindrical cavity with poor lighting and visibility. Also used, on someoccasions, are tungsten burr drills, assorted reeming drills, morerecently experimental laser cutting devices, and alternative surgicaltechniques which involve making a "window" in the bone in a positioncorresponding to where it is thought the prosthetic cement finishes andthen removing the cement from below.

These techniques are poor in that they are of long duration andgenerally place the live bone in risk of damage. Damage, such asfacture, can be a serious problem because many of the patients areelderly.

DISCLOSURE OF THE INVENTION

In one broad form the present invention may be said to provide atemplate profiled so as to correspond to the profile of the line ofintersection of a generally longitudinal pre-selected plane of a jointprosthesic cavity and the associated prosthetic cement/bone interface ofa patient.

An alternative broad form of the invention can be said to provide amethod of forming a guide means for use in removing prosthetic cementfrom a bone of a patient comprising the steps of determining the profileof the prosthetic cement/bone interface along a predetermined linerunning longitudinally of the bone and forming said guide means adaptedto guide a suitable cutting means such that it cuts through the cementalong said lines substantially without cutting into the bone.

Broadly speaking the invention can also be said to provide a method ofproducing a template, such as the template described above, includingthe steps of:

scanning the patient in the area of the joint prosthesis so as to obtaina profile of the prosthetic cement/bone interface at its intersectionwith the longitudinal pre-selected plane of the cavity; and

feeding the profile into a machine which, in response thereto, forms thetemplate being profiled correspondingly to the profile of the line ofintersection of the pre-selected plane and the cement/bone interface.

Preferred apparatus of the invention can be said to comprise:

the template as described above;

a linear saw guide being rigidly attached to, or integral with, thetemplate;

a saw adapted for reciprocating cutting action and traversable along theguide for linear advancement of the cutting action; and

a template follower urging the saw to cut along a path beingsubstantially the line of intersection of the pre-selected plane and thecement/bone interface.

In a particular preferred embodiment the template may be the base of agroove of a saw guide.

In another particular preferred embodiment the template may be the sawwhich is advanced transversely of the guide, in which case no templatefollower is required.

Another broad form of the invention may be said to provide a method oflongitudinally cutting prosthetic cement in a bone of a patient, usingthe above-described apparatus, and comprising:

positioning the guide and template fixedly relative to the prostheticcavity approximately in the longitudinal pre-selected plane associatedwith the template profile;

introducing the saw cooperatively to the guide;

commencing reciprocating cutting action of the saw, and

advancing the saw such that a groove is cut into the prosthetic cementto a depth approximately equal to the depth of the cement andsubstantially without cutting into the bone.

In a preferred method of the invention a number, usually 3 or 4, oflongitudinal cuts are produced in the prosthetic cement spaced apartabout the cavity and produced by the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an artificial joint prosthesis fixed in abone;

FIG. 2 schematically shows in longitudinal cross-section a bone in whichthe prosthetic cement has been cut;

FIG. 3 schematically shows equipment used in producing devices of theinvention;

FIGS. 4 and 5 schematically show two slightly differing embodiments ofthe invention;

FIG. 6 schematically shows an alternative device embodying theinvention;

FIG. 7 schematically shows a further device embodying the invention, thedevice being shown in place within a bone;

FIG. 8 is similar to FIG. 7 but shows an alternative device embodyingthe invention;

FIG. 9 schematically shows an alternative device, somewhat similar tothe devices of FIGS. 4 and 5, which also embodies the invention;

FIG. 10 schematically shows another alternative embodiment of theinvention; and

FIG. 11 shows schematically the device of FIG. 10 in cooperation with ablade in use in carrying out a method embodying the invention.

BEST MODE OF CARRYING OUT THE INVENTION

As seen in FIG. 1 the femoral prosthesis stem is held in place in thefemur of a patient by a cement, which is normally polymethylmethacrylate(PMMA). If the prosthesis has to be removed for any reason it cannormally be pulled relatively easily from the cement leaving a closedend tubular cement plug within the bone which must then be removed toallow a fresh prosthesis to be inserted and re-cemented.

In the process of removing the prosthetic cement, the present inventionallows it to be cut to the depth of the cement in a longitudinal linewithout cutting into the bone by any substantial amount. Normally 3 or 4of such cuts are made so as to be spaced apart around the perimeter ofthe plug.

It is well known to use X-ray machines linked with computer analysisequipment in order to produce three-dimensional representations ofhidden bones etc. Such machinery and associated skills can be adapted tocontrol the cutting machine. The cutting machine produces a templatewhich has a face or edge which is shaped so as to correspond to thecontour of the line of intersection of the prosthetic cement and thebone at a pre-selected generally longitudinally running plane of thejoint prosthesic cavity. The correspondence of the template profile tothat of the actual cement/bone interface will depend upon such aspectsas the device which will be used to follow the template profile, thegeometry of the guiding means and saw during advancement of the saw andthe cutting head intended for use in carrying out the operation. Theremight, for example, be a scaling factor between the actual bone/cementinterface shape and the shape of the template.

In broad terms the cutting operation is carried out by guiding areciprocating saw blade within the prosthetic cement cavity, the finaldepth of cut along the cutting controlled by the profile of a templateso as to cut entirely through the cement without substantially cuttingthe bone.

Frequently a bottom plug of the prosthetic cement is simply bored outusing conventional equipment. The axis of this boring should be alignedwith the cuts which are to be made so that the cuts can continue intothe bored out section.

Particular embodiments of the invention will now be discussed in detail.

A guide blank 1 is shaped substantially the same as the stem of theprosthesis which is to be removed as shown in FIG. 4. The guide blank 1is machined by the cutting device 2 under the control of the equipmentshown in FIG. 3 and as generally described above. The cutting machine 2produces a groove 3 within the guide 1, the depth of the groove 3varying along its length so as to be of substantially the same profileas the cement/bone interface at the intersection of the correspondingpre-selected plane. The groove 3 is cut in a position within the guide 1so that upon insertion of the guide 1 into the prosthesic cavity, formedafter the removal of the prosthesis being discarded, it aligns with thepredetermined plane. A short suitably shaped surgical gradereciprocating saw blade head, with a suitable follower adapted to alignthe saw head with the groove 3 and to follow the profile of the base ofthe groove 3, can be introduced into the groove 3 and gradually advancedtherealong while being actuated by a suitable reciprocating device.

In some circumstances the saw head can be initially inserted, followedby the guide 1 and the cutting action advanced from the cavity interiortowards the exterior.

It is possible to use, in a similar fashion to that described above, aguide 1 which is a close fit but somewhat smaller that the stem of thediscarded prosthesis. Such a guide 1 is shown in FIG. 5 in place in abone and secured by a wedge 4. One advantage of this embodiment is thatthe exact size of the stem of the prosthesis to be discarded does nothave to be known before commencing the removal operation.

In FIG. 6 the guide 1 is seen to have a fluid tight bladder 5 which isattached on the side opposite the groove 3. The guide 1 can be placed inthe cavity of the cement to be removed before fluidly pressurising thebladder 5. The pressurized bladder 5 then rigidly fixes the guide 1within the cavity.

Clearly in the above described guides the template is an integral partof the guide being the profile of the base line of the guide groove 3.However, the template may be quite separate from the guide.

An embodiment having separate guide and template is shown in FIG. 7. Atemplate 6 is produced by a similar method as used in producing theguide 3 of the previous embodiments, using the equipment described withrelation to FIG. 3. The template 6 is rigidly attached to a base 7 whichis secured by suitable means to the end of the bone 11 containing thecement 10 to be removed. The saw includes a saw head 8 on the end of awand 9 which is attached to a suitable reciprocating device. A portionof the wand 9 includes a template follower 15. The wand 9 is guided by aguide device 14 which may be spring biased or the wand 9 itself may beresiliently flexible. In this embodiment the profile of the template 6may be approximately a mirror image of the cement/bone interface profileat the line being cut depending upon the relative positions of thedevice 14 and the follower 15.

A further embodiment with an external template separate to the guide isshown in FIG. 8. A lever 26 is pivotally attached to the end of a rigidarm 27. The arm 27 is advanced through stationary guides 14. Thetemplate follower 15 is also pivotally attached to the arm 27, thetemplate follower 15 and lever 26 being coupled by tie rod 16. Thetemplate follower 15 is biased by a tensioned spring 17. The template 6is arranged relative to the guides 14 such that the template follower 15advances with the arm 27 along the template, the angle between thefollower 15 and the arm 27 altering in response to the profile of thetemplate 6. Such alteration of angle of the template follower 15 isreflected in an angular movement of the lever 26 causing movementtransverse of the arm 27 corresponding to profile changes in thetemplate 6 so that it follows the desired cement/bone interface. The sawhead 8 is attached to an end of the wand 9 and maintained alongside thelever 26 so as to be biased by the lever 26 toward the cement during thecutting action obtained by reciprocative movement of the wand 9. The cutwill progress transversely into the cement until the follower 15 comesup against the template 6. The template 6 and guides 14 are rigidly heldby a grooved guide similar to that shown in FIG. 4. In this case thebase of the groove 3 is flat.

FIG. 9 shows a guide holder 17 which is fixedly positioned approximatelycentrally of the cavity 13 by suitable adjustment of the various screws18. Suitable guides having grooved templates similar to the grooves ofthe guides 1 of FIGS. 4 and 5 can be attached to the support 17 and usedto produce a corresponding cut 12 in the cement 10. The support 17 couldalso be used to position devices such as those described in FIGS. 7 and8.

An alternative type of guide 19 is shown in FIG. 10. A system of spacers20 are attached to the exterior of the guide 19 so as to hold it in adesired position within the cavity 13. The guide 19 includes a generallylongitudinally running groove 21 with flanges 24 partially closing offthe transverse entrance of the groove 21.

FIG. 11 shows the guide 19 of FIG. 10 situated within a cavity 13 (thebone 11 cement 10 combination being cross-sectioned in the Figure), incooperation with a suitable saw 22. The cutting edge of the saw 22 isprofiled in the manner of the templates 6 of the embodiment shown inFIGS. 7 and 8. The profiled saw is produced by a method similar to thatdescribed with reference to FIG. 3 above. The base 23 of the saw 22 isinitially proximate the base of the groove 21 of the guide 19. Afterpositioning the blade 22 longitudinally in the guide 19 it isreciprocated by a suitable device and advanced transversely of the guide19 so as to simultaneously cut along the entire length of the desiredcut 12. When the base 23 of the saw 22 reaches the flanges 24 it isprevented from further transverse movement. The dimensions of the groove21 and saw 22 are pre-selected so that at this position the saw hascompleted the cut and cut 12 has progressed completely through thecement 10 but not substantially into the bone 11.

I claim:
 1. A template having a profile corresponding to a profile of aline of intersection of a generally longitudinal pre-selected plane of ajoint prosthetic cavity and an associated prosthetic cement/boneinterface of a patient, the template comprising a guide shaped as a stemof a prosthesis to be replaced in the patient, and including alongitudinally running groove positioned in a plane of the guidecorresponding in orientation to the pre-selected plane and including agroove base with a profile within said plane being the template profile.2. A method of forming a guide means for use in removing prostheticcement from a bone of a patient at a prosthetic cement/bone interfacecomprising the steps of determining a profile of the prostheticcement/bone interface along a predetermined line running longitudinallyof the bone and forming said profile on said guide means for guiding asuitable cutting means such that it may cut through the cement alongsaid line substantially without cutting into the bone.
 3. A productproduced by carrying out the method of claim
 2. 4. Apparatuscomprising:a template having a profile corresponding to a profile of aline of intersection of a generally longitudinal pre-selected plane of ajoint prosthetic cavity and an associated prosthetic cement/boneinterface of a patient; a linear saw guide being rigidly attached to, orintegral with, the template; a saw adapted for reciprocating cuttingaction and traversable along the guide for linear advancement of thecutting action; and a template follower connected with the saw forurging the saw to cut along a path being substantially the line ofintersection of the pre-selected plane and the cement/bone interface. 5.Apparatus as defined in claim 4 wherein the guide is shaped as a stem ofa prosthesis to be replaced in the patient, and including alongitudinally running groove positioned in a plane of the guidecorresponding in orientation to the pre-selected plane and including agroove base with a profile within said plane being the template profileand the apparatus further comprises wedges adapted to firmly secure bywedging action the guide within a prosthetic cavity around which thecement is to be removed.
 6. Apparatus as defined in claim 4 furthercomprising a base on the template and adapted so that the base is firmlyattachable to a bone end in which prosthetic cement is to be removed andwith the template and bone on opposite sides thereof, the base includingan aperture for insertion of said saw, and wherein said saw being a sawhead at one end of a wand and the opposite end of the wand including anattachment for connection to a reciprocating device and said saw guideis rigidly attachable to the base and positioned and shaped so as toguide said saw wand through said aperture during longitudinaladvancement of the saw.
 7. Apparatus as defined in claim 6 wherein saidwand is elastically transversely flexible.
 8. A method of producing atemplate including the steps of:scanning a patient in an area of a jointprosthesis so as to obtain a profile of a prosthetic cement/boneinterface at its intersection with a longitudinal pre-selected planethrough a medullary cavity in the bone; and feeding the profile into amachine which, in response thereto, forms the template with a surfacehaving said profile.
 9. A method of removing prosthetic cement from abone of a patient comprising producing a template by scanning a patientin an area of a joint prosthesis so as to obtain a profile of aprosthetic cement/bone interface at its intersection with a longitudinalpre-selected plane through a medullary cavity in the bone; feeding theprofile into a machine which, in response thereto, forms the templatewith a surface having said profile, removing the prosthesis to bediscarded leaving a prosthetic cavity, cutting at least onelongitudinally running cut in the prosthetic cement withoutsubstantially cutting the bone by using said surface as a cutting guideso as to form a number of arcuate prosthetic cement parts and removingeach cement part.
 10. A method for removing prosthetic cement from abone of a patient undergoing a joint prosthesis replacement operation,comprising the steps of:(a) producing a template profiled so as tocorrespond to the profile of the line of intersection of a generallylongitudinally pre-selected plane of a joint prosthesis cavity and anassociated cement/bone interface of the patient by,(i) scanning thepatient in the area of the joint prosthesis so as to obtain a profile ofthe said prosthetic cement/bone interface at its intersection with thesaid pre-selected plane, and (ii) forming on a template blank a templatesurface having a profile corresponding to the profile of the said lineof intersection, (b) removing the prosthesis to be replaced from theprosthetic cavity, (c) positioning the template in relation to theprosthetic cavity for guiding a cutting blade along the template to forma running cut substantially completely through the prosthetic cementalong the said line of intersection and substantially without cuttingthe adjacent bone and guiding the blade along the template to form saidcut, (d) replacing at least once the steps (a) and (c) along the line ofintersection of one or more other generally longitudinal pre-selectedplanes of the joint prosthesis cavity and the associated cement/boneinterface of the patient to form a number of segmented prosthetic cementpieces, and (e) removing the segmented prosthetic pieces from the bone.11. A method as claimed in claim 10 in which the scanning of the patientis carried out in an X-ray machine.
 12. A method as claimed in claim 10in which a bottom plug of the prosthetic cement is bared out before therunning cuts are formed in the prosthetic cement.
 13. A method asclaimed in claim 10 in which at least three running cuts are formed inthe prosthetic cement and the cuts are substantially equiangularlyspaced around the prosthetic cavity.